i also have a yz 250 2002 which isn't working ( my klx never seams to have this problem haha)
i just had i rebuilt after seizing it in the dunes. i went for a ride last weekend and started it up and went fine once it warmed up a bit rode it like 50 meters and it ran like **** and putted out, like it had fouled the plug. so tried to start it again no luck so pulled plug out and it was black as hell so cleaned it up. but when i started it again it fired first pop ran for like 5 sec then putted out. so tried to start again no luck pulled plug out fouled again. so in the garage it went i just had a look at it now and with out all my gear on and other bikes going when i went to start it ( with brand new spark plug in it) it fired for a few strokes then stopped but i could hear air puffing out and on closer look there was a hole in the exhaust pipe. just a tinny one on the header pipe just about 10 cm out from the motor as the pipe doubles on its self. would this be why its not starting and ruining? it sounded like a lot of air was coming out ie alot of pressure lost.

 

Improper scavenging of an engine. Get that exhaust leak fixed, then let us know.

edit: first thoughts anyways.

 

thanks ill take it into work and see if one of the welders can fix it and let you know.

 

hi got it welded and bikes going great now. took it for a great trail ride on sunday and didn't miss a beat. thanks for the help.